Many vehicles are used over a wide range of vehicle speeds in both forward and reverse. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. When the vehicle is at high speed, the transmission is usually operated at a low speed ratio to permit the engine to facilitate quiet, fuel efficient cruising.
To ease driver workload, many automotive transmissions have automatic controls that select among the available speed ratios and engage the correct clutches or brakes to establish the selected speed ratio. For example, as illustrated in FIG. 1, a transmission controller may select a gear ratio based on accelerator pedal position and vehicle speed. The transmission upshifts when the current condition crosses a shift line from left to right. An upshift may be triggered either because the vehicle speed has increased as shown at 10 or because the driver has reduced the accelerator pedal position. When the current condition crosses a shift line from right to left, either due to vehicle slowing or to an increase in the pedal position, the transmission downshifts as shown at 12. To avoid excessive shifting when operating near the shift point, the downshift line may be offset slightly to the left of the corresponding upshift line. Sudden dramatic changes in accelerator pedal position may cause the current condition to cross completely over one of the regions as shown at 14. In such a situation, the vehicle controller may shift directly to the destination ratio without ever engaging the intervening ratio. Such multi-step shifts can either upshifts or downshifts.
Sometimes, drivers wish to exert control over the selected transmission ratio while still enjoying the convenience of automatic shifting most of the time. For example, a driver planning to accelerate in a few seconds may wish to command a downshift in preparation for the acceleration event as opposed to waiting until the transmission controller recognizes that the acceleration event is occurring. Some manufacturers have provided additional controls such as paddles mounted beside the steering wheel which a driver can use to manually command an upshift or downshift of an automatic transmission. These extra controls add cost to the vehicle and are often inconvenient for drivers to use while turning the steering wheel.